Method for adaptive target processing in a motor vehicle radar system

ABSTRACT

The invention relates to a method for adaptive target processing in a vehicle radar, the targets in the surroundings being detected by speed and place in the standard mode of the radar sensor, after detection of the targets in the standard mode it is being changed over to a precision mode, in which the distance measuring range of the radar sensor is adapted to the target surroundings detected in the standard mode. In accordance with the present invention the measuring accuracy and/or resolution regarding speed is increased by increasing the time of observation within the distance measuring range adapted to the target surroundings.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a method for adaptive target processingin a vehicle radar, the targets in the surroundings being detected byspeed and place in the standard mode of the radar sensor, afterdetection of the targets in the standard mode it is being changed overto a precision mode, in which the distance measuring range of the radarsensor is adapted to the target surroundings detected in the standardmode.

[0003] 2. Description of the Related Art

[0004] Usually the radar sensors used in automobiles are used for thecontrol of speed in traffic surroundings such as country roads ormotorways. The frame conditions for such application are such thatinformation on the surroundings are necessary for a distance range ofapprox. 10 m to 150 m. However, for city traffic or stop-and-go it is amust to measure as far as to a short distance ahead of the vehicle (<2m).

[0005] In addition, there are specific traffic situations, e.g. two carswith nearly identical speed, in a surpassing action in the detectingrange of the radar antennas, in which the necessary target separation isnot attained to a sufficient extent. Such cases require a secondarytreatment, e.g. in a topped tracking filter. The tracking filter servesfor tracking the single, Individual targets detected by the radar deviceover the time.

[0006] With radar systems the data cycle time, i.e. the time requiredfor detecting the complete measuring range, is defined by the processingtime for range finding and Doppler measurement. With the popular systemsfor distance control in a vehicle this time is defined such that theymeet the user requirements regarding accuracy of distance and speedmeasurement, while accepting the above-cited disadvantages.

[0007] By DE 44 33 775 A1 a method for adaptive target processing in avehicle radar is disclosed, in which with regard to the distance it ischanged over between a normal mode and a precision mode with an extendedresolution. This method forms the preamble of claim 1.

SUMMARY OF THE INVENTION

[0008] It is an object of the invention to introduce an adaptive signalprocessing before the tracking filter, which permits to cover the veryshort range as far as just ahead of the antenna and to further improvethe target separation.

[0009] This object is attained by the method for adaptive targetprocessing in a vehicle radar, the targets in the surroundings beingdetected by speed and place in the standard mode of the radar sensor,after detection of the targets in the standard mode it is being changedover to a precision mode, in which the distance measuring range of theradar sensor is adapted to the target surroundings detected in thestandard mode. Advantageous embodiments are subject to further claims.

[0010] In accordance with the present invention a precision mode withextended resolution and/or increased accuracy of measurement regardingspeed and if necessary distance is introduced subsequent theabove-described standard mode, in which the radar sensor detects targetsby speed and distance with the resolution and measuring accuracy whichis common for vehicle applications. Simultaneously, the distancemeasuring range of the radar is adapted to the target surroundingsdetected in the standard mode, i.e. in the precision mode merely arestricted distance measuring range is taken into consideration.

[0011] In case e.g. in the standard mode the detection shows a target orseveral targets in a given distance range, specifically this distancerange can be detected within the precision mode and that with anextended speed resolution and/or speed measuring accuracy. In anadvantageous embodiment in addition the distance resolution and/or thedistance measuring accuracy can be increased.

[0012] Also cases are possible in which within the distance rangeadapted to the target surroundings merely the speed resolution and/orspeed measuring accuracy is increased in the precision mode, thedistance resolution and/or the distance measuring accuracy remainingconstant.

[0013] In a particularly advantageous embodiment adaption of thedistance measuring range while simultaneously increasing the resolutionand/or measuring accuracy is performed in such manner that the datacycle time and thus the data renewal rate for the precision mode remainsconstant compared to the standard mode.

[0014] Consequently, the method according to the invention correspondsto the application of adaptive filters In signal processing, whichfilters may be adapted to the respective situation.

[0015] Thus an improved target separation and a very short rangedetection can be attained exclusively by altering the signal processing,without additional constructional measures being required at the radarsensor.

[0016] The invention will become apparent from the ensuing descriptionof examples of embodiment taken in conjunction with the drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017]FIG. 1 shows an illustration of the distance gates and of theDoppler processing with a Doppler radar system. The axis pointing upwarddesignates the time axis for the Doppler processing. T_(D) designatesthe processing time for the Doppler determination. Fs is the Dopplersampling rate and 1/fs the temporal distance between two Doppler samples(serial no. 1, . . . n in FIG. 1). Typical values are: number n of theDoppler samples=64; 1/fs=20 μs. With a complex target data processingand for the frequency range of the vehicle radars the following resultsfrom these sizes:

[0018] Speed measuring range: +/−175 km/h

[0019] Speed resolution: ˜5 km/h.

[0020] The horizontal axis designates the time axis for the individualdistance gates. T_(R) is the time for a data cycle, i.e. the time whichis required to detect the complete measuring range. A typical value forT_(R) is 1 μs. T_(E) designates the width of a distance gate ETk withk=1, 2, . . . , m. For application in a vehicle radar T_(R) is usuallysmaller than T_(D).

[0021] Common vehicle radars classify the distance measuring range intodefined distance gates ETk, in which the speeds of the detected objectsare determined with preset Doppler band widths. As an example: Thelength of the distance gates is approx. 5 m and the speed resolution isin the range of 5 km/h.

[0022] Several distance gates may be processed to blocks (width of ablock: T_(Ri)) combined in the Doppler level, the Doppler processing forthe distance gates of a block being effected parallel in time. For thispurpose, e.g. a multi-channel receiver may be used. The time T_(R) issubdivided into time segments T_(Ri), which must equal the conditionthat

T_(Ri)<1/fs,

[0023] the sampling frequency fs of the Doppler processing having to fitShannon's theorem for the maximum occurring Doppler frequency. Thesubdivision into time segments T_(Ri) is advantageous in view of thelimited storage place in the signal processing of the radar and permitsoptimization of the processing time (calculating time fortransformations). In this way, for instance, when subdividing T_(R) into8 time segments T_(Ri) the complete data cycle can be processed in atime T_(Dat) of

T_(Dat)=8*T_(D).

[0024] With the method according to the present invention upon knowledgeof the total surroundings—after detection of targets in the standardmode—it is changed over to a distance range, which corresponds to one ormore time segments T_(Ri), by having discovered the targets relevant forthe control and which covers a part of the original distance range.

[0025] By extending the time of observation T_(D) in this partial rangeand—optionally—by reducing the length of the distance gate ETkimprovements with regard to resolution and/or measuring accuracy as tospeed and if necessary distance can be attained. The relevant targetscan now be examined at a closer look or can be more easily separated.Moreover, multiple reflexions, which in particular in the very shortrange lead to measuring inaccuracies, can be better distinguished fromdirect reflections by the increased resolution/measuring accuracy.

[0026] With the reduction of the distance range with a simultaneouslyincreased resolution and/or measuring accuracy for speed and ifnecessary distance a constant data renewal rate can be attained, thuspermitting at any time a change over between both modes (total range inthe standard mode—target range in the precision mode).

[0027] The two concepts subject to the present invention, namelyextension of the time of observation in the Doppler determination andreduction of the length of the distance gates are further describedhereinafter.

[0028] 1. Reduction of the Length of a Distance Gate ETk

[0029] With puls radars the length of a distance gate T_(E) usuallycorresponds to the transmit puls length τ. The distance resolution withpuls radars can be improved by reducing τ. Moreover, it is also possiblewith puls radar systems to attain an improvement in distancedetermination by reducing the length of distance gate T_(E) whilemaintaining τ, however, at the cost of energy balance. Therefore, thisembodiment is important for major targets for the very short range (highecho field intensity) or in the distant range.

[0030] In case of FM-CW radars (FMCS: frequency modulated-continuouswave) the frequency deviation ΔF of the HF-signal defines the length ofthe distance gates, the individual distance gates being determined byband pass filters. Reduction of the length of the distance gates can beattained by increasing the frequency deviation ΔF. For this purposeprincipally two options are available:

[0031] 1) by alteration of the upgrade of the frequency rise of theHF-signal (in case of triangle or sawtooth modulation) with an unalteredduration of a modulation period, or

[0032] 2) by extending the duration of the modulation period of theHF-signal with an unaltered upgrade of the frequency rise.

[0033] 2. Extension of the Time of Observation T_(D)

[0034] Usually Doppler processing is performed after temporalintegration of the echo signals (serial no. 1, . . . , n in FIG. 1)within the same distance gate ETk. With a puls radar from each puts asample is gained for the Doppler processing. With a FM-CW radar fromeach modulation period a sample is gained for the Doppler processing.

[0035] After scanning of the echo signals integrated within a distancegate ETk in the time domain with a sampling frequency fs a Fouriertransform is performed on the scanned values. This yields the spectralrepresentation of the Doppler signal, from which the Doppler frequencyf_(D) of the target can be detected, which interrelates with the targetspeed via

v_(Z)=f_(D)*A₀/2 with A₀=wave length of the HF-signal.

[0036] The temporal length of the scanning

T_(D)=n/fs

[0037] defines the Doppler resolution Δf_(D)

Δf_(D)=1/T_(D).

[0038] With the method according to the present invention in theprecision mode while maintaining the sampling rate fs the duration ofthe scanning T_(D) can be extended and thus the filter band width in theDoppler processing can be reduced, i.e. the speed resolution can beimproved by increasing the number n of the samples. The same effect ofan extension of T_(D) is attained by reducing the sampling frequency fsand maintaining the number n of the samples. However, reducing thesampling frequency fs results in a reduction of the uniquely detectablespeed range.

What is claimed is:
 1. A method for adaptive target processing in avehicle radar, the targets in the surroundings being detected by speedand place in the standard mode of the radar sensor, after detection ofthe targets in the standard mode it is being changed over to a precisionmode, in which the distance measuring range of the radar sensor isadapted to the target surroundings detected in the standard mode,wherein the measuring accuracy and/or resolution regarding speed isincreased by increasing the time of observation within the distancemeasuring range adapted to the target surroundings.
 2. A method as setforth in claim 1, wherein the data renewal rate for the precision moderemains unaltered compared to the standard mode.
 3. A method as setforth claim 1 or 2, wherein the adaptation of the distance measuringrange to the target surroundings detected in the standard mode goesalong with an increase of the number of the Doppler samples with anunaltered Doppler sampling rate.
 4. A method as set forth in claim 1 or2, wherein the adaptation of the distance measuring range to the targetsurroundings detected in the standard mode goes along with a reductionof the Doppler sampling rate with an unaltered number of the Dopplersamples.
 5. A method as set forth in one of the preceding claims 1 to 4,wherein the measuring accuracy and/or resolution regarding distance isincreased by reducing the length of the distance gates within thedistance measuring range adapted to the target surroundings.
 6. A methodas set forth in claim 5, wherein a puls radar is used as a radar sensor,the adaptation of the distance measuring range to the targetsurroundings detected in the standard mode going along with a reductionof the length of the distance gates while reducing the transmit pulselength.
 7. A method as set forth in claim 5, wherein a puls radar isused as a radar sensor, the adaptation of the distance measuring rangeto the target surroundings detected in the standard mode going alongwith a reduction of the length of the distance gates with a constanttransmit pulse length.
 8. A method as set forth in claim 5, wherein aFM-CW radar is used as a radar sensor, the adaptation of the distancemeasuring range to the target surroundings detected in the standard modegoing along with an increase of the transmitter frequency deviation. 9.A method as set forth in claim 8, wherein the increase of thetransmitter frequency deviation is performed by increasing the upgradeof the frequency rise within a modulation period with an unalteredduration of the modulation period.
 10. A method as set forth in claim 8,wherein the increase of the transmitter frequency deviation is performedby extending the duration of the modulation period with an unalteredupgrade of the frequency rise.
 11. Application of the method inaccordance with one of the preceding claims for suppressing multiplereflections in the very short range of the radar sensor.